Industrially, tert-butylamine is a very important intermediate for making dyes, pesticides, and rubber additives.
The industrial processes are based on the so-called Ritter reaction in which, by addition of hydrocyanic acid and water to olefins or by the substitution reaction of alcohols with hydrocyanic acid and water, a formamide derivative is first formed, hydrolysis of which leads to the desired amine. Sulfuric acid is added in nearly equimolar quantities as a catalyst. Formic acid is produced as a by-product. The process involves toxic and aggressive substances, the handling, separation, and reprocessing of which are technologically and financially intensive.
The addition of ammonia to isobutene on which the present invention is based is an equilibrium reaction and has been investigated for a very long time without leading to an industrial process. Because of the position of the equilibrium, only very low conversions, less than 20%, are achieved under industrial reaction conditions. Economic recycling of the unreacted educt is achievable only with very high selectivity for tert-butylamine. A method based on the addition reaction therefore has to be characterized by high product selectivity, long catalyst life, and high catalyst activity.
A number of attempts have been made in the past to develop an industrially usable process from the addition reaction, but thus far it has proved impossible to develop such a process that meets all the requirements described briefly below.
An industrially usable process is characterized above all by minimizing the ratio between recycling streams and product streams. This means using equimolar amounts of ammonia and isobutene. It is important, however, for this step taken to optimize the recycling ratio not to diminish the selectivity of the catalyst for tert-butylamine.
Such an industrially usable process must however also be able to tolerate variations in concentration, namely a deficiency of ammonia should not increase formation of by-products (oligomerization and cracking of isobutenes, and formation of di(tert-butyl)amines).
In the previously known processes for manufacturing tert-butylamine, however, the ammonia:isobutene ratio was 1.5:1 or more.
Two contradictory demands are made in the first instance on the catalyst candidate for the reaction: The catalyst should have high activity to come as close as possible to the equilibrium conversion, but it should not promote by-product formation and coking (high selectivity).
Also, the catalyst should be sufficiently reactive at temperatures below approximately 320.degree. C., as otherwise the reactants and reaction products would be thermally decomposed and increased coking or by-product formation would occur.
In a recycling process usable on an industrial scale, suppression of by-products such as highly volatile amines (methylamine) is very important since these could otherwise be fed back into the reaction as low boilers and could there react to form mixed amines (such as tert-butylmethylamine). After several cycles, the proportion of these by-products would increase continuously so that the yield of tert-butylamine would be inadequate and the cost of producing pure (greater than 99.5%) tertbutylamine would be increased dramatically by expensive separation operations.
Thus, it must be possible to operate an industrial process with technical raw materials without costly pretreatment. In general, such raw materials also have certain proportions of impurities which of course are expected to be inert in the recycle loop and have no negative effect on the activity/selectivity and life of the catalyst, and can easily be separated.
In particular, n-butene, which is normally present in technical isobutene (educt) in small quantities, should not be reacted with ammonia to form isobutylamine, as otherwise the product specifications (highly pure (&gt;99.5%) tert-butylamine) are not met. In this case as well, the recycling process described (e.g. TBA/isobutylene separation) would be heavily burdened by expensive separation stages.
In principle, synthesis of tert-butylamine from ammonia and isobutene is known for example from EP-A 0 039 918 where good selectivities can be achieved with comparatively low conversion, but where selectivity declines as conversion increases. Because of their fast coking, the catalysts used in this process have only short lives.
If, as may be gathered from DE-OS 33 26 579, the prior-art catalyst is selected for long life, i.e. a low degree of coking, tert-butylamine selectivities of less than 98.7% are achieved. The gallium silicate zeolite referred to in DE-OS 39 40 349 also has selectivity of only approximately 98%; in this case, however, very high pressures are required for quantitative conversion.
In a number of other prior-art processes as well, because of the stoichiometric equilibrium, by-products arise in the classical substitution reaction which have to be removed in expensive purification steps.
A goal of the invention is to provide a method with which tert-butylamine can be manufactured in a simple and environmentally benign manner, and in which no by-products that would have to be expensively removed from the reaction mixture are produced. In addition, the catalyst should have a long operating life as well as high selectivity and activity.